1. Technical Field
The present disclosure relates to, for example, a secondary battery tester for testing the state of a secondary battery on the basis of its impedance characteristic.
2. Related Art
To determine whether or not a secondary battery has been manufactured normally, a self-discharge test (shelf test) is performed immediately after the manufacture. In this test, a test subject battery is charged to a prescribed voltage V1 and a voltage V2 is measured after it has been left as it is for a prescribed time Δt. Furthermore, a good/bad determination is performed by estimating a self-discharge amount on the basis of a thus-obtained voltage fall ratio (V1−V2)/Δt. Whereas in many cases a full-charging voltage is employed as the voltage V1, a voltage that is lower than the full-charging voltage may be used as the voltage V1 depending on the characteristics of a battery.
However, at room temperature, self-discharge proceeds very slowly and hence the voltage variation rate is very low, as a result of which about two weeks are necessary for detection of a clear voltage reduction. Thus, the self-discharge test is a bottle neck of a test process. A self-discharge test is sometimes conducted in a shorter time in such a manner that a battery is left as it is in a very low voltage range approximately corresponding to an SOC 0% where a relatively fast voltage variation occurs and its temperature is kept high to accelerate the self-discharge. However, leaving a battery as it is in a very low voltage range may cause an overdischarge state, and a discharge at a high temperature may accelerate deterioration of the battery (an originally non-defective battery may be damaged). In there circumstances, development of an improved self-discharge test method, in particular, a method capable of determining, simply in a short time, whether or not a battery is defective in self-discharge performance, is desired.
On the other hand, methods for detecting a defect in a secondary battery are disclosed in Japanese Patent Documents JP-A-2003-100351, JP-A-2009-145137, JP-A-2003-317810 and JP-A-2000-299137, for example. JP-A-2003-100351 discloses a technique of detecting deposition of metal ions on the basis of a voltage variation in charging that is done at the initial stage of manufacture. JP-A-2009-145137 discloses a technique of detecting a difference in voltage variation from a standard battery. JP-A-2003-317810 discloses a technique of making a determination “abnormal” if reaction resistance is small. JP-A-2000-299137 discloses a technique of determining an initial activity characteristic on the basis of impedance of a nickel-hydrogen battery. However, none of these conventional techniques can properly evaluate the state of an SEI (solid electrolyte interface) layer that is formed on the negative electrode surface of a lithium ion secondary battery. In lithium ion secondary batteries, such abnormalities as a self-discharge defect occur depending on the state of an SEI layer. Therefore, none of the conventional techniques can properly determine whether a lithium ion secondary battery is good or bad. Lithium ion secondary batteries have a characteristic that the impedance varies to a large extent with the charging factor. Although there should exist a proper charging factor for recognition of the state of an SEI layer, the conventional techniques do not refer to this point.
Impedance values of secondary batteries measured in a self-discharge test process have a large variation and this measurement is low in reproducibility.
Also, in a self-discharge test process, the voltage of a secondary battery should be kept close to, for example, a voltage corresponding to full charging because a secondary battery is damaged if it is discharged excessively.